Add internal fdt_cells() to avoid copy and paste. Fix typo in
fdt_size_cells() documentation comment.

This is based in upstream commit:
c12b2b0 ("libfdt: fdt_address_cells() and fdt_size_cells()")
but misses the test cases, as we don't implement them in U-Boot.

Signed-off-by: Matthias Brugger <mbrugger@suse.com>
Reviewed-by: Simon Glass <sjg@chromium.org>